1. Field of the Invention
The present invention generally relates to an apparatus and method for receiving a signal in a communication system, and more particularly to an apparatus and method for receiving a signal by selecting whether to use interference cancellation according to a modulation scheme in a communication system.
2. Description of the Related Art
Since limited resources such as frequency, code and timeslot resources, are divided and used in multiple cells of a communication system with a cellular structure (hereinafter cellular communication system), Inter-Cell Interference (ICI) may occur.
When the frequency resources are divided and used in the multiple cells of the cellular communication system, the ICI results in performance degradation. The frequency resources are reused to increase the overall capacity of the cellular communication system. Herein, the rate at which the same frequency resources can be reused is referred to as a “frequency reuse factor”. The frequency reuse factor is defined by the number of cells in which the same frequency resources are unused. Assuming that the frequency reuse factor is 1/K, the number of cells in which the same frequency resources are unused becomes K.
As the frequency reuse factor is small, that is, when the frequency reuse factor is less than 1, ICI decreases but an amount of frequency resources available in one cell decreases. Thus the overall capacity of the cellular communication system also decreases. In contrast, when the frequency reuse factor is 1, that is, all of the cells in the cellular communication system use the same frequency resources, ICI increases but an amount of frequency resources available in one cell increases. Thus the overall capacity of the cellular communication system also increases.
Substantial research is being conducted on next generation communication systems for providing users with services based on various classes of Quality of Service (QoS) at a high transmission rate. Among the next generation communication systems, a typical communication system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16e communication system, which is a cellular communication system.
FIG. 1 illustrates a structure of the conventional IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16e communication system has a multi-cell structure, that is, a cell 100 and a cell 150, and includes a Base Station (BS) 110 covering the cell 100, a BS 140 covering the cell 150, and multiple Mobile Stations (MSs) 111, 113, 130, 151 and 153.
The IEEE 802.16e communication system as illustrated in FIG. 1 has a frequency reuse factor of 1. In this case, an amount of frequency resources available in one cell increases along with the efficiency of frequency resources. However, since frequency resources, that is, sub-carriers, are the same between a serving BS and a neighbor BS in a cell overlap area, ICI may occur, degrading the performance of signal reception from the serving BS in an MS located in the cell overlap area.
To compensate for this degradation, the IEEE 802.16e communication system applies a most robust Modulation and Coding Scheme (MCS) level available therein, modulates and codes MAP information, and transmits the modulated and coded MAP information. Herein, all the BSs of the IEEE 802.16e communication system use the same most robust MCS level. The MAP information includes control information such as position information regarding downlink and uplink burst regions, modulation scheme information, and allocation information of the downlink and uplink regions, that is, information regarding whether the downlink and uplink burst regions are dedicatedly allocated to a specific MS or are commonly allocated to unspecific MSs. For example, the IEEE 802.16e communication system modulates and codes the MAP information at a Quadrature Phase Shift Keying (QPSK) ½ level and then transmits the MAP information after a maximum of six repeats.
The reception performance of the MAP information in the MS located in the cell overlap area may not be improved to a level desired in the IEEE 802.16e communication system even when the MAP information is transmitted at the most robust MCS level available in the IEEE 802.16e communication system. Thus, to eliminate the ICI the IEEE 802.16e communication systems use special interference cancellation schemes such as successive interference cancellation (SIC).
The performance of the SIC scheme depends on a range of a Signal to Interference and Noise Ratio (SINR). For example, as the SINR is low, that is, a size of an interference signal is large, the performance of the SIC scheme is superior. In contrast, as the SINR is high, that is, the size of the interference signal is small, the performance of the SIC scheme is inferior. Thus a scheme for selecting whether to use the SIC scheme according to an SINR (hereinafter Norm SIC scheme) has been proposed to eliminate the performance degradation in an SNR range of the SIC scheme.
The Norm SIC scheme uses a ratio between the channel powers of a serving BS and of a neighbor BS as a measure of the SINR. When a measured SINR is less than a threshold SINR, that is, an interference signal can be correctly measured, a control operation is performed such that the SIC scheme is used. In contrast, when the measured SINR is equal to or greater than the threshold SINR, that is, the interference signal cannot be correctly measured, a control operation is performed such that the SIC scheme is unused. However, the Norm SIC scheme may ensure sub-optimal performance only when the same modulation scheme as that of the neighbor BS is used, that is, the same modulation scheme is applied to a desired signal and an interference signal, as when all the BSs of the IEEE 802.16e communication system transmit MAP information. Herein, the neighbor BS transmits the interference signal in a frequency domain, that is, a sub-channel, equal to that in which the desired signal is transmitted. The sub-channel includes at least one sub-carrier.
However, when the IEEE 802.16e communication system conventionally transmits traffic data, modulation schemes applied to the desired signal and the interference signal are usually different. The above-described Norm SIC scheme may ensure optimal performance when the modulation schemes applied to the desired signal and the interference signal are the same. However, when different modulation schemes are applied to the interference signal and the desired signal as in the traffic data, the performance level may not be ensured. In the current IEEE 802.16e communication systems, an MS may not detect information regarding a modulation scheme applied to traffic data of a neighbor BS since the MS receives only MAP information of a serving BS without receiving MAP information of the neighbor BS.